We have recently succeeded in generating for the first time mammalian cell lines containing functional suppressor tRNA genes. The nonsense mutant-nonsense suppressor system is an attractive genetic system because: 1) it is a conditional lethal system, 2) nonsense mutants, in the absence of a suppressor exhibit extremely low activities, 3) the gene product of a nonsense mutation is readily identified and 4) using a battery of nonsense suppressors a series of amino acids can be inserted at the same site in a protein. In this grant we propose a series of projects which exploit and/or extend this genetic system in higher eucaryotes including measuring the efficiency of suppression in the newly isolated cell lines, creating cell lines containing different levels of suppression, creating cell lines which insert different amino acids at the site of a nonsense mutation and isolating mammalian cell lines containing endogenous suppressors.